Add IPMaskToString function

v0.2.2

Change return value for IPMaskToString from *string to string

New functions, changed some parameters to pointers

This release brings two new functions: IsIPv4Net and IncrIP, and change some arguments and return values to pointers.

Warning! All net.IP and net.IPNet agruments and return values were changed to pointers to be more consistent between all functions. Update your code accordingly.

Initial release

v0.1.0

Add go.mod